The present invention relates to an apparatus and a method for restraining response of a structure to outside disturbances such as winds, earthquakes etc.
One conventional method for making structures resistant against such vibrational loads as earthquakes, winds and traffics is to construct the structure enough strong to withstand the external loads. But recently, a notion so called a seismic isolation has been developed and brought into practical use for protecting the structure from seismic loads. The seismic isolation is to support the structure by a soft and flexible support and absorb the seismic load by a large deformability of the support. A problem as to the seismic isolation is, though the isolation is effective for cutting off vibrational loads which is to be induced through the support, that it sometimes makes the situation worse when a vibrational load is induced directly to the building without passing through the supports. Such situations occur when the structure vibrates because of winds or, for example, when a bridge vibrates because of the traffic passing thereon. In such cases, vibration is apt to become larger if a conventional seismic isolation technique is applied.
In order to resolve these inconveniences, present inventors have already proposed a vibration restraining apparatus for buildings as described in Japanese patent application 62-241045. Japanese Patent Application, discloses a vibration restraining apparatus comprising a tank and a liquid retained in the tank to be installed on the roof of a building so as to restrain vibration of the building. As the building vibrates, vibrational energy of the building excites the liquid, having a sloshing natural frequency identical to a natural frequency of the building, to slosh in resonance with the building. Consequently, vibration of the building is restrained as the vibrational energy of the building is transmitted to the liquid. It was stated in the specification that the weight of the liquid retained in the tank had better be larger than 1% of the building so as to assure an effectiveness.
A problem as to the above-mentioned apparatus and methods for restraining vibration of a building is that the apparatus has a very narrow band of resonance. Therefore, the apparatus acts only when it is excited at its natural frequency exactly. However, it is very difficult to determine the dimensions of the tank and the amount of the liquid so that the natural frequency of sloshing exactly resonate with the vibration of the building. Further, if the natural frequency of the structure shifts by deterioration of the material for example, the apparatus becomes "detuned" and thus rendered ineffective for its intended purpose.
Another problem is that sloshing of the liquid continues long because the damping coefficient in sloshing is normally far less than that of ordinary structures. Therefore, after external excitation such as winds and earthquakes is terminated, the liquid continues to slosh and the liquid becomes a source of vibration to exite the structure.